U.S. Defense Secretary Ash Carter is telling NATO leaders the alliance must improve its ability to defend itself against cyberattacks before it tries to build its cyber warfare capabilities.

Carter's remarks in Brussels come amid rising tensions with Russia, which has proven its willingness to launch computer-based attacks against other nations.

His message on offensive cyber operations runs counter to some experts and leaders who believe NATO should begin to develop cyber weapons, in order to deter opponents in the 21st century.

Senior defense officials say cyber defense was one theme of Carter's remarks to the allies Wednesday, and to defense ministers he's met with in recent days. The officials spoke on condition of anonymity because they were not authorized to discuss the matter publicly.

No Cold War-style arms race

Meanwhile, NATO Secretary General Jens Stoltenberg says the alliance will not be forced into a Cold War-style arms race with Russia.

He told reporters in Brussels Wednesday that the alliance must upgrade its military capabilities in order to defend member-states.

"We will not be dragged into an arms race. But we must keep our countries safe," he said. " We will also work very closely with partners to help keep our neighborhood stable," Stoltenberg said.

Stoltenberg's comments followed the disclosure by U.S. Defense Secretary Ash Carter that the United States will pre-position about 250 tanks, along with other heavy weapons and military equipment, in six eastern European NATO countries.

Stoltenberg said Wednesday he supported the U.S. decision to pre-position weaponry and provide other forms of military assistance to the NATO members in the east - Estonia, Lithuania, Latvia, Bulgaria, Romania and Poland.

"I welcome the decision by the United States both to pre-position equipment but also to provide key capabilities - as, for instance, air-to-air fueling, special operation forces, strategic airlift. As I said, this is defensive; this is something which is prudent and a necessary response to what we have seen from Russia over a long period of time," Stoltenberg said.

President Vladimir Putin said last week that Moscow will add at least 40 inter-continental ballistic missiles to its nuclear arsenal this year.

Rapid Response Force

On Monday, the NATO secretary general said the alliance will more than double the size of its rapid-response force - from its current level of 13,000 personnel to up to 40,000 troops.

Pentagon chief Carter is on a weeklong tour of Europe focused on U.S.-Europe cooperation in the face of Russia's reported direct support to separatists in eastern Ukraine.

In response to repeated allegations by Ukraine about Russian activities in eastern Ukraine, Moscow has consistently denied that it has sent troops or arms to the rebels.

Carter and the other NATO ministers are meeting through Thursday on plans for dealing with Russia's actions in Ukraine. Speaking earlier in Tallinn, Estonia, the U.S. defense secretary said the Western powers are not trying to be Russia's enemies. However, he added, the alliance will defend itself if necessary.